## 3 Civilizations Now: 1

In the example of the calculation in Table 1, the fraction of planets where civilization evolves out of the earliest forms of life has been put at 0.0001 (one in ten thousand) and the lifetime of such a civilization is put at 5,000 years

**Table 1: Sample Drake Equation
Calculation**

Stars per Year | 20 |

Planets per Star | 1 |

Fraction in Habitable Zone | 0.2 |

Fraction with Life | 0.5 |

Fraction with civilization | 0.0001 |

Civilization Lifetime, years | 5,000 |

Present Number of Civilizations |
1 |

**Present Number of Civilizations
= 20 x 1 x
0,2 x 0.5 x 0.0001 x 5000 = 1**

On the basis of these arbitrary numbers (Assumptions 1), we are the only civilization present at this time (but how many civilizations have existed in the past has not been calculated.). If the civilization lifetime were 500 years, the number of civilizations would be 0.1, which can be interpreted as the probability that one civilization exists now, or that one civilization would appear in the course of a decade. If civilization lifetimes averaged five million years, there would be 1000 civilizations. Changing the fraction of civilizations emerging from primitive life affects the estimate in a similar way.

It is a very useful feature of the Drake Equation that civilization lifetime combines so easily with the other factors to give civilization numbers. It works because the multiplication by years converts the per-year figures to a pure number. However, the per-year calculations that have been carried out in previous years are still valid and suggest that many civilizations have flourished in the past but are no longer in existence. The implications can be seen by examining the calculation in more detail.